In a typical cellular network, also referred to as a wireless communication system, User Equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks (CNs).
A user equipment is a mobile terminal by which a subscriber can access services offered by an operator's core network. The user equipments may be for example communication devices such as mobile telephones, cellular telephones, laptops or tablet computers, sometimes referred to as surf plates, with wireless capability. The user equipments may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
User equipments are enabled to communicate wirelessly in the cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the cellular network.
The cellular network covers a geographical area which is divided into cell areas. Each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also on cell size.
A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some radio access networks, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or eNBs, may be directly connected to one or more core networks.
UMTS is a third generation, 3G, mobile communication system, which evolved from the second generation, 2G, mobile communication system GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
In the context of this disclosure, a base station as described above will be referred to as a base station or a Radio Base Station (RBS). A user equipment as described above, will in this disclosure be referred to as a user equipment or a UE.
The expression DownLink (DL) will be used for the transmission path from the base station to the user equipment. The expression UpLink (UL) will be used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
Cellular communication networks evolve towards higher data rates, together with improved capacity and coverage. In 3GPP, standardization body technologies like GSM, HSPA and LTE have been and are currently developed.
LTE uses a radio access technology based on Orthogonal Frequency Division Multiplexing (OFDM) for downlink transmissions and based on Single Carrier Frequency Division Multiple Access (SC-FDMA) for uplink transmissions. The resource allocation to user equipments on both downlink and uplink may be performed adaptively by the concept of so called fast scheduling, taking into account the instantaneous traffic pattern and radio propagation characteristics of each user equipment. Assigning resources in both downlink and uplink may be performed in a scheduler situated in a base station, which base station is in LTE referred to as an eNodeB.
A problem is that it may be difficult for the base station to schedule the transmissions to obtain satisfactory quality of service in the cellular network. For example, one challenge for the scheduler is to maximize the throughput for the user equipments experiencing the worst channel quality and at the same time achieve a high total throughput.
As cellular systems typically operate on a specific limited bandwidth due to for example cost of licenses, manufacturing, etc. it is therefore highly desirable to utilize the spectrum as efficiently as possible. This has mainly led to the introduction of the so called reuse-1 in the most modern cellular systems such as e.g. LTE, and Worldwide Interoperability for Microwave Access (WiMax). A reuse-1 means that all of the available licensed spectrum is reused in all cells in the system, and is performed in order to increase the system's capacity. However, a reuse-1 would inevitably lead to a very high interference for cell-edge users, mainly in the uplink due to power limitation of the user equipments.
Several approaches have been proposed to alleviate this limitation. One general approach is reuse partitioning, wherein a reuse-1 is utilized for cell-center user equipments, as they are not so susceptible to inter-cell interference, and a so called reuse-3 is utilized for cell-edge user equipments. Reuse-3 means that the reuse distance is equal to 3 i.e. the same frequency resources are reused every third cell.
In reuse partitioning, the spectrum, that is the frequency resources, is explicitly divided into two parts: cell-center spectrum which consists of the same resources for all cells, and cell-edge spectrum. The cell-center spectrum is allocated to cell-center users in all cells with a reuse-1 fashion. The cell-edge spectrum is allocated to cell-edge users in a reuse-3 fashion.
In so called Soft Frequency Reuse (SFR), the reuse-3 of cell edge users is obtained in a ‘soft’ fashion, i.e. by letting it overlap with the cell-center spectrum of the other cells. The reasoning behind such an approach is that cell-center users are rather immune to inter-cell interference so they would not be affected by being scheduled on the same resources as cell-edge users in neighboring cells, and that the cell-center users will not generate high interference to the cell-edge users either.
A problem is however, that such reuse partitioning schemes have limitations since they result in unutilized parts of the spectrum and too little allocation for cell-edge users. Therefore, such reuse partitioning schemes are not very useful to balance between good cell capacity and cell-edge user throughput in the cellular network.